I have a couple of baby lock machines...I've had that issue and the guy I bought mine from said to use canned air to blow out any frabic dust from the tension dial(s). This has helped sometimes.
Another thing that I have had to do is set my sewing machine bobbin tension by using the weights from my embroidery professional machine.
I don't know if this helps but I do know as much as I love my babylocks and brother machines that this is my main frustration with them.